Chronic pneumonia in swine has been recognized as a major problem in swine production for almost a century. The disease has a high morbidity with low mortality causing a chronic cough, dull hair coat, retarded growth and unthrifty appearance lasting several weeks. Characteristic lesions of purple to gray areas of consolidation particularly in ventral apical and cardiac lobes are observed in infected animals. Death may result from secondary infection or stress. One of the causes of chronic pneumonia is infection by Mycoplasma hyopneumoniae. Economic losses alone have been estimated at between 200 to 250 million dollars annually.
The bacteria was first identified in 1965. Mycoplasma hyopneumoniae is a slow growing, fastidious bacterium which lacks a cell wall. It is frequently difficult to isolate from the respiratory tract due to Mycoplasma hyorhinis, a common secondary agent also located in the respiratory tract.
The disease is spread by aerosol, produced by coughing, and by direct contact from an affected or convalescent carrier swine. Mingling of infected animals and uninfected animals results in early and frequent reinfection. Infection frequently starts with infection of piglets by carrier sows at farrowing. Because of current herd management techniques, infection may remain silent until later in life. Additional infection usually is observed after weaning when pigs are pooled. Overt disease is normally observed in pigs at six weeks of age or older. Average animal growth rates are reduced by about 16% with feed conversion rates being reduced by about 22%.
Surveys of slaughtered animals revealed lesions typical of Mycoplasma pneumonia in 30-80% of swine. Results from 337 herds in 13 states indicated that 99% of the herds had hogs with pneumonia lesions typical of Mycoplasma pneumonia. Therefore, the need for effective preventative and treatment measures are great.
Tiamulin, trimethoprim, tetracyclines and lincomycin have been shown to have some benefit. However, antibiotics are expensive and require prolonged use. Additionally, reinfection is an ever present problem. Antibiotics have not been shown to effectively eliminate spread of Mycoplasma hyopneumoniae. Prevention by maintaining pathogen free herds is possible but reintroduction of Mycoplasma hyopneumoniae occurs.
Vaccines generally employ one of four categories of antigens: live microorganisms administered via an unnatural route, live attenuated microorganisms, killed microorganisms and fractions or even a single antigen or product of a microorganism. In all situations, the goal is to present antigens without giving the disease. A number of different inactivating agents and means have been employed including formalin, azide, freeze-thaw, sonication, heat treatment, sudden pressure drop, detergent (especially non-ionic detergents), lysozyme, phenol, proteolytic enzymes and .beta.-propiolactone.
Thimerosal has been proposed for inactivation of certain bacteria unrelated to Mycoplasma in Greenberg et al, U.S. Pat. No. 3,522,790 and Muggleton et al, 2,908,614. Ragland et al, U.S. Pat. No. 5,004,607 inactivates their bacteria with formalin (column 4, line 59) but also later adds Thimerosal as a preservative. Thimerosal has not previously been used to inactivate Mycoplasma hyopneumoniae for vaccine use.
RespiSure vaccine sold by Smith Kline Beecham is a chemically inactivated Mycoplasma hyopneumoniae vaccine administered with an oil adjuvant. Petersen et al; Proc. Amer. Assoc. Swine Pract., Mar. 1991, p. 17-21 discloses inactivating Mycoplasma hyopneumoniae with formalin to produce a bacterin for potential vaccine use. Faulds et al, U. S. Pat. No. 4,985,243 uses freeze-thaw to extract antigens from Mycoplasma hyopneumoniae in order to prepare a vaccine.
Recombinant DNA techniques have been used to express an antigen which had potential for use as a vaccine. Schaller et al, U. S. Pat. No. 4,894,332.
Mycoplasma hyopneumoniae has been lysed and analyzed in terms of its proteins, for example, Wise et al, Journal of Bacteriology, 169: 5546-5555 (1987). However, it has not been established which one or ones are the critical proteins which must be recognized to elicit a protective immune response.
A number of vaccines in the past have used adjuvants to enhance the immunogenicity of an antigen. The mechanism of how adjuvants operate is not entirely known. Some are believed to enhance the immune response by slowly releasing the antigen while other adjuvants are strongly immunogenic in their own right and are believed to function synergistically. Among the adjuvants which have been used in various vaccines include, oil and water emulsions, complete Freund's adjuvant, incomplete Freund's adjuvant, Corynebacterium parvum, Hemophilus, Mycobacterium butyricum, aluminum hydroxide, dextran sulfate, iron oxide, sodium alginate, Bacto-Adjuvant, certain synthetic polymers such as poly amino acids and co-polymers of amino acids, saponin, iota carrageenan, "REGRESSIN" (Vetrepharm, Athens, Ga.), "AVRIDINE" (N, N-dioctadecyl-N',N'-bis(2-hydroxyethyl)-propanediamine), Mannite monooleate, paraffin oil, and muramyl dipeptide.
Yoshioka et al, U.S. Pat. No. 3,917,819, used an aluminum hydroxide gel as an adjuvant in a formalin-killed Mycoplasma suipneumoniae vaccine, while Dayalu, Proceedings: Mycoplasma Pneumonia Symposium, Smith Kline Beecham, 1-15 (1990) states on page 12 that Kobisch et al (1987) used aluminum hydroxide in a Mycoplasma hyopneumoniae vaccine. Kishima et al, Veterinary Microbiology, 13: 335-42 (1987) have used dextran sulfate as an adjuvant in an azide-killed mycoplasma vaccine. Outside the field of bacterial vaccines, Roumanian Patent 75,100 has proposed a viral vaccine using aluminum hydroxide and DEAE dextran as an adjuvant. However, this combination has not been used to our knowledge with any bacteria, let alone as an adjuvant with Mycoplasma.
Regardless, the goal of a vaccine is to provide protection against natural infection. A detectable immune response, such as producing detectable quantities of antibodies, may not necessarily be protective. Thus, while vaccines have been attempted to protect swine from infection by Mycoplasma hyopneumoniae, acceptable levels of protection have not been achieved.